An Internal Combustion Engine (ICE) is a cyclic device in which the thermal energy of burned fuel is partially converted into mechanical energy. The fundamental upper limit for efficiency of such a conversion is set by the efficiency of the reversible Carnot cycle operating between the same limiting hot (TH) and cold (TC) temperatures as the real engine, η=1−TH/TC (as is well-known, this statement represents one of the equivalent formulations of the Second Law of Thermodynamics). For example, an engine operating between the combustion temperature of gasoline (TH=2,300 K) and room temperature (TC=300 K), the Carnot efficiency η is 87%. Conventional ICEs such as Otto cycle engines have efficiencies at best approaching 40-50% using low octane rated fuels available for consumers. Various attempts have been made to improve the efficiency of internal combustion engines but much room for increased efficiency remains.
As is well-known, gasoline-fueled ICE operation can be described as an idealized, cyclic process called the Otto cycle. The Otto cycle is depicted in FIG. 1A in terms of its different stages (strokes), in FIG. 1B on a thermodynamic pressure-volume diagram, and in FIG. 2A as a pressure vs. cycle phase diagram.
The Otto cycle stages include:
1) Intake stroke: taking in an amount of gas (air mixed with fuel); during this stroke the volume of a cylinder increases by a factor which is called a compression ratio r; for present-day automobile ICE, r is equal to about 9-10.5.
2) Compression stroke: quickly compressing the mixture to a considerably higher pressure while raising its temperature. This compression takes place almost without heat exchange with the cylinder walls, i.e. near-adiabatically, but at the same time in an almost-reversible way; in an idealized Otto cycle this is represented by an adiabatic lower curve on a PV diagram in FIG. 1B. The maximum pressure at point b in FIG. 1B to which the mixture can be compressed is limited by the condition that it does not self-ignite (the undesirable phenomenon of self-ignition is also called detonation or dieseling),
3) Ignition: intentionally igniting (e.g., via a spark plug) the compressed mixture at point b in FIG. 1A, creating a nearly instantaneous increase in pressure at a fixed volume (i.e., an isochoric process b-c in FIG. 1B, followed by
4) Power stroke: a near-adiabatic and almost-reversible expansion of combustion products by a volume factor r, accompanied by the partial transfer of thermal energy of the burned mixture into mechanical energy; the transfer being achieved via setting in motion, by the gas pressure, an appropriate mechanical element such as a moving piston, as shown in FIG. 1A (e.g., or a rotor in a rotary ICE).
5) Exhaust stroke: The moving element (e.g. a piston) pushes out the used burned mixture through the open exhaust valve while lowering the pressure back to atmospheric level (another isochoric process d-a in FIG. 1B)
Thus, there is a need for systems and methods which more efficiently convert the internal thermal energy of expanding burned fuel mixture in an internal combustion engine into mechanical energy.